1. Field of the Invention
This invention relates to a process of forming a container by biaxial-orientation blow-molding a cylindrical shaped preform of thermoplastic resin mainly comprising acrylonitrile polymers and, more particularly, to a process of forming a container in which 0.1 ppm or less of residual acrylonitrile monomer is included.
2. Related Art
A container obtained by biaxial-orientation blow-molding a cylindrical shaped preform of thermoplastic resin mainly comprising acrylonitrile polymers has excellent gas barrier properties, excellent permeability, light weight and excellent stability against a number of kinds of solvents. Accordingly, the container is widely utilized as for, for example, filling food, medicines, toiletries and the like.
Acrylonitrile polymers contain trace amounts of residual acrylonitrile monomer. Trace amounts of residual acrylonitrile monomer are also produced in steps of molding and heating the preform to form the container. Accordingly, residual acrylonitrile monomer is contained in the blow-molded container.
Japanese Patent Publication No. 56-54321 and Japanese Patent Laid-open No. 54-148059 disclose a method of reducing the amount of residual acrylonitrile monomer in a preform of thermoplastic resin mainly comprising acrylonitrile polymers by irradiating the preform with an electron beam.
The method of reducing he residual acrylonitrile monomer in the preform by irradiating the preform with the electron beam has a disadvantage in that HCN is a by-product of the electron beam irradiation. Therefore, this method cannot provide a container made of acrylonitrile resin having excellent sanitary properties.